One of the biggest mysteries to come out of The Force Awakens was probably the identity of the Knights of Ren. Though the group does look to be from the Dark Side, they are definitely not the Sith. Star Wars Explained goes in-depth as to their possible origin, and how the Knights rose to power 30 years after the death of the Emperor.
The latest Aftermath novel, Empire's End, has delved further into a group of Dark Side fanatics called the Acolytes of the Beyond. The acolytes were initially introduced as a nonviolent group, but in Aftermath: Life Debt, the group attacked a precinct and Corellia and spread graffiti on the walls saying "Vader Lives."
The organization is also known to have a deep obsession with Dark Side artifacts, and Vader seems to be in the center of their beliefs. They also have a deep emphasis on masks, believing that they can imbue powers to the wearer.
The real question, however, is how the group will eventually morph into the Knights of Ren. It's even speculated that the Knights aren't even force-sensitive and that Kylo was able to become their leader because of his powers in the Force and his relation to Darth Vader. Not to mention, all of them wear masks, and Kylo is in possession of Vader's burnt helmet.
Of course, it all boils down to speculation, but the Acolytes of the Beyond seem to be building up towards the eventual reveal of who the Knights of Ren really are. Hopefully the answers come out in The Last Jedi.
Star Wars: The Last Jedi is set for a release on Dec. 15.